Radiation image conversion panel

ABSTRACT

A radiation image conversion panel comprising a phosphor sheet having a stimulable phosphor layer and a protective layer provided so as to cover the surface of the phosphor layer, wherein the average slant angle Δa of the surface roughness of the outer surface of the protective layer not adjacent with the phosphor layer is from 0.01 to 0.1 and the surface roughness Ra in μm of the inner side or the phophor sheet side of the protective layer is 0.05 μm to 0.45 μm, and the stimulable phosphor layer is provided by a coating method.

FIELD OF THE INVENTION

The present invention relates to a radiation image conversion panelemploying a stimulable phosphor, and relates in detail to a radiationimage conversion panel having a high resistivity against heat, by whichan image with high sharpness and without unevenness of image density canbe obtained.

BACKGROUND OF THE INVENTION

Radiographic images such typically as X-ray images are widely employedin various fields, for example, in the medical diagnosis. To obtain theX-ray image, radio-photographic method is principally utilized in whichradiation penetrated through a subject is irradiated to a phosphorlayer, also called as a phosphor screen, and light emitted from thephosphor layer is irradiated to silver halide photographiclight-sensitive material and then the light-sensitive material isdeveloped to obtain a visible image. Recently, however, new method fordirectly obtaining the image from the phosphor layer is proposed insteadof the image forming method by the light-sensitive material.

In such the method, radiation penetrated through the subject is absorbedby a phosphor and then the phosphor is stimulated by light or thermalenergy so as that the energy accumulated in the phosphor by absorbingthe radiation is released as fluorescence and the fluorescence isdetected and imaged. In concrete, the method is a radiation imageconversion method utilizing a stimulable phosphor such as that describedin U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open toPublic Inspection, hereinafter referred to as Japanese Patent O.P.I.Publication, No. 55-12144.

In this method, a radiation image conversion panel containing thestimulable phosphor is utilized. In detail, the radiation imageconversion panel is exposed to radiation penetrated through the subjectso that the radiation energy corresponding to the radiation transmissionof each parts of the subject is accumulated in the stimulable phosphor,and then the energy of the radiation accumulated in the phosphor isemitted as light by time serially stimulating the phosphor bystimulating electromagnetic radiation such as visible light and infraredrays. The signal according to the variation of the emitted light isreadout, for example, as electric signals by photo-electric conversion,and the electric signals are reproduced as a visible image on a usualimage recording medium such as the light sensitive material or an imagedisplaying apparatus typically such as a CRT.

The foregoing reproduction method for reproducing the radiographic imagerecord has an advantage such as that a radiographic image with abundantinformation by an exposure amount of radiation considerably smaller thanthat to be required in the radio-photographic method employing acombination of the radiographic light-sensitive material and anintensifying screen.

On the outer surface of the stimulable phosphor layer, namely thesurface not to be faced to the support, a protective layer forprotecting the phosphor layer from a chemical deterioration or aphysical impact. As the protective layer, the followings have beenknown: one formed by coating on the phosphor layer a solution composedof an organic polymer such as a cellulose derivative and poly(methylmethacrylate) dissolved in a suitable solvent; one formed by adhering aseparately prepared protective layer forming sheet or film composed ofan organic film such as polypropylene and poly(ethylene terephthalate)or a glass plate on the phosphor layer by a suitable adhering agent; andone formed by depositing an inorganic compound by a method such as vapordeposition.

The stimulable phosphor is a phosphor which emits light when irradiatedby stimulating light after exposed to radiation. The phosphor emittinglight of from 300 to 500 nm by stimulating light of from 400 to 900 nmis usually employed in practice. Examples of the stimulable phosphoremployed hitherto in the radiation image conversion panel arealkali-earth metal fluoride-halide compound type phosphors eachactivated by a rare-earth element described on Japanese Patent O.P.I.Publication Nos. 55-12145, 55-160078, 56-74175, 56-116777, 57-23673,57-23675, 58-206678, 59-27289, 59-27980, 59-56479 and 59-56480;di-valent europium-activated alkali-earth metal halide type phosphorsdescribed in 59-75200, 60-84381, 60-106752, 60-166379, 60-221483,60-228592, 60-228593, 61-23679, 61-120882, 61-120883, 61-120885,61-235486 and 61-235487; rare-earth element-activated oxohalidephosphors described in Japanese Patent O.P.I. Publication No. 59-12144;cerium-activated tri-valent metal oxohalide phosphors described inJapanese Patent O.P.I. Publication No. 58-69281; bismuth-activatedalkali halide type phosphors described in Japanese Patent O.P.I.Publication No. 60-70484; di-valent europium-activated alkali-earthmetal halo-phosphate phosphors described in Japanese Patent O.P.I.Publication Nos. 60-141783 and 60-157100; di-valent europium-activatedalkali-earth halo-borate phosphors described in Japanese Patent O.P.I.Publication No. 60-157099; di-valent europium-activated alkali-earthhydrogen-halide phosphors described in Japanese Patent O.P.I.Publication No. 60-217354; cerium-activated rare-earth composite halidephosphors described in Japanese Patent O.P.I. Publication Nos. 61-21173and 61-21182; cerium-activated rare-earth halo-phosphate phosphorsdescribed in Japanese Patent O.P.I. Publication No. 61-40390; di-valenteuropium-activated cerium rubidium phosphors described in JapanesePatent O.P.I. Publication No. 60-78151; di-valent europium-activatedhalogen phosphor described in Japanese Patent O.P.I. Publication No.60-78153; and rare-earth metal-activated tetradecahedron alkali-earthmetal fluoride-halide type phosphors precipitated from a liquid phasedescribed in Japanese Patent O.P.I. Publication No. 7-233369.

Among the above-described stimulable phosphors, the di-valenteuropium-activated iodide-containing alkali-earth metal fluoride-halidetype phosphors, the di-valent europium-activated iodide-containingalkali-earth metal halide type phosphors, the rare-earth metal-activatediodide-containing rare-earth metal oxohalide type phosphors, and thebismuth-activated iodide-containing alkali metal halide type phosphorseach emit high luminance light.

It is one of the advantages of the radiation image conversion panelemploying that the panel can be repeatedly usable because such thephosphor emits by scanning by the stimulating light and radiographicimage can be re-accumulated thereafter. Such the method is alsoadvantageous from the viewpoint of the resources protection and theeconomical efficiency since the radiation image conversion panel can berepeatedly usable contrary to that the radiographic light-sensitivematerial is consumed every once of the photographing in the usualradiographic method.

As described above, though the radiographic image recording-reproducingmethod has various advantages, it is demanded to the image conversionpanel to be used in such the method that the sensitivity and the imagequality such as the sharpness and the graininess are as higher aspossible.

In the performance of the method, the radiation image conversion panelis repeatedly employed in the circle of exposure to radiation forrecording the radiographic image, irradiation of the stimulating lightfor reading the recorded radiographic image and irradiation of erasinglight for erasing the remained radiographic image. In a cassette typepanel, the panel is conveyed through each steps by a conveying meanssuch as a belt and a roller to perform such the repeatedly use of thepanel. On the occasion of such the operation, the temperature of theinterior of the apparatus is made higher than the room temperature bythe presence of the light source and the driving system. In the case ofan exclusive type apparatus, the temperature interior of the apparatustends to be raised higher than that of the cassette type apparatus byheating by the light source in the course of continuous image takingeven though there is no panel conveying means in the apparatus. It hasbeen known that the unevenness of image density is degraded when heat isapplied. Consequently, high resistivity against heat is required to thepanel.

On the other hand, it has been considered that the sharpness of theimage obtained by the radiation image conversion panel is higher whenthe thickness of the protective layer is thinner. However, thedurability of the conversion panel tends to be lowered because thescratches and the cracks on the protective layer surface tend to beformed in the course of the repeatedly use when the thickness of theprotective layer is thin. Therefore, suitable materials are elected sothat the functions as the protective layer such as anti-scratch propertyand the conveying suitability can be maintained even when the thicknessof the protective layer is made as small as possible for inhibiting thedegradation of the sharpness. For example, the use of a material havinga high strength and transparency such as poly(ethylene terephthalate),combination of plural kinds of resins, coating of a layer of acomposition containing fluorine-containing resin having highanti-scratch ability and the use of multi-layered protective layer areapplied.

However, the thickness of the protective layer should be increased inthe usual technology to obtain the durability for prolonged period inthe apparatus so that degradation of the sharpness is resulted.

Problems that unevenness of the density other than the density variationcaused by the subject occurs when a film of polypropylene, poly(ethyleneterephthalate) or poly(ethylene naphthalate) is used as protectivelayer, or the density unevenness caused by the coating of the resincomposition containing fluorinated resin occurs. As the countermeasureto such the density unevenness, Japanese Patent O.P.I. Publication No.59-42500 and Japanese Examined Patent Publication No. 1-57759 disclosesa measure to disappear the density unevenness by raising the haze ratioof the protective layer. However, a drawback is caused that theincreasing of the haze ratio results further lowering of the sharpness.

Consequently, a method described in, for example, Japanese Patent O.P.I.Publication No. 10-82899, in which the protective layer is constitutedby a plastic film and a fluorinated resin-containing resin compositionwhich contains light scattering fine particles coated on the film, and amethod described in, for example, Japanese Patent O.P.I Publication No.2002-122698, in which the protective film has a stimulating lightabsorbing layer and specified surface roughness, are disclosed. However,the improvement of the sharpness is insufficient and the compatibilityof high sharpness and low density unevenness cannot be satisfied yet.

SUMMARY OF THE INVENTION

An object of the invention is to provide a radiation image conversionpanel having high thermal resistivity and giving a radiographic image ofhigh sharpness without density unevenness.

The above-described object can be attained by the followings.

1. A radiation image conversion panel comprising a phosphor sheet havinga stimulable phosphor layer and a protective layer provided so as tocover the surface of the phosphor layer, wherein the average slant angleΔa of the surface roughness of the outer surface of the protective layernot adjacent with the phosphor layer is from 0.01 to 0.1 and the surfaceroughness Ra in μm of the inner side or the phosphor sheet side of theprotective layer is 0.05 μm to 0.45 μm, and the stimulable phosphorlayer is provided by a coating provided type.

2. The radiation image conversion panel described in 1, wherein theprotective layer comprises a stimulating light absorbing layer beingcolored to absorb the stimulating light.

3. The radiation image conversion panel described in 1, wherein a binderto be used in the stimulable phosphor layer is selected from proteinssuch as gelatin, polysaccharide such as dextrin, natural polymers suchas gum arabic, and synthesized polymers such as poly(vinyl butyral),poly(vinyl acetate), nitrocellulose, ethyl cellulose, vinylidenechloride-vinyl chloride copolymer, poly(alkyl acrylate), poly(alkylmethacrylate), vinyl chloride-vinyl acetate copolymer, polyurethane,cellulose acetate butylate, poly(vinyl alcohol) and linear polyester.

4. The radiation image conversion panel described in 3, wherein thebinder is selected from nitrocellulose, linear polyester, poly(alkylacrylate), poly(alkyl methacrylate), a mixture of nitrocellulose andlinear polyester, a mixture of nitrocellulose and poly(alkyl acrylate)or poly(alkyl methacrylate) and a mixture of polyurethane and poly(vinylbutyral).

5. The radiation image conversion panel of claim 1, wherein the ratio ofthe amount of the binder to be used in the stimulable phosphor layer tothat of the stimulable phosphor is within the range of from 0.03 to 0.2parts by weight per one part by weight of the stimulable phosphor.

6. The radiation image conversion panel described in 1, wherein thepanel is composed of the phosphor sheet cut into a designated size andmoisture proof films provided on both sides of the phosphor sheet andthe films are each substantially not adhered with the phosphor sheet andthe circumferences thereof are arranged at the outside of thecircumference of the phosphor sheet so as to cover the entire surface ofthe phosphor sheet.

7. A radiation image conversion panel comprising a phosphor sheet havinga stimulable phosphor layer and a protective layer provided so as tocover the surface of the phosphor layer, wherein the average slant angleΔa of the surface roughness of the outer surface of the protective layernot adjacent with the phosphor layer is from 0.01 to 0.1 and the surfaceroughness Ra in μm of the inner side or the phosphor sheet side of theprotective layer is 0.05 μm to 0.45 μm, and the stimulable phosphorlayer is provided by a gas phase accumulation type.

8. The radiation image conversion panel described in 7, wherein theprotective layer comprises a stimulating light absorbing layer beingcolored to absorb the stimulating light.

9. The radiation image conversion panel described in 7, wherein thestimulable phosphor constituting the stimulable phosphor layer is alkalihalide type stimulable phosphor represented by Formula 1:M1X.aM2X′₂.bM3X″₃:eA   Formula 1wherein M1 is an alkali metal selected from the group consisting of Li,Na, K, Rb and Cs; M2 is a divalent metal selected from the groupconsisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu and Ni; M3 is a trivalentmetal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga and In; X, X′ and X″ areeach a halogen selected from the group consisting of F, Cl, Br and I; Ais a metal selected from the group consisting of Eu, Tb, In, Cs, Ce, Tm,Dy, Pr, Ho, Nd, Yb, Er, Gd, Lu, Sm, Y, Tl, Na, Ag, Cu and Mg; and a, band e are each a value within the range of 0≦a<0.5, 0≦b<0.5 and 0<e≦0.2,respectively.

10. The radiation image conversion panel described in 9, wherein thestimulable phosphor constituting the stimulable phosphor layer isrepresented by Formula 2:CsX:A   Formula 2in which X is Br or I, and A is Eu, In, Ga or Ce.

11. The radiation image conversion panel described in 10, wherein thestimulable phosphor constituting the stimulable phosphor layer is CsBrtype phosphor.

12. The radiation image conversion panel described in 7, wherein thepanel is composed of the phosphor sheet cut into a designated size andmoisture proof films provided on both sides of the phosphor sheet andthe films are each substantially not adhered with the phosphor sheet andthe circumferences thereof are arranged at outside of the circumferenceof the phosphor sheet so as to cover the entire surface of the phosphorsheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the situation of formation of a stimulable phosphor layeron a support by vapor deposition.

FIG. 2 shows a schematic drawing an example of method for forming thestimulable phosphor layer on the support by the vapor deposition.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in detail below.

As the protective film according to the invention, polyester film,polymethacrylate film, nitrocellulose film, cellulose acetate film areemployable, and film such as polypropylene film, poly(ethyleneterephthalate) film and poly(ethylene naphthalate film are preferable asthe protective film from the viewpoint of the transparence and thestrength. As the fluorinated resin-containing resin composition layer, alayer of polymer of fluorine-containing olefin or fluoro-olefin and thatof copolymer containing the fluorine-containing as the copolymeringredient are preferable from the viewpoint of the anti-scratchability.

In the invention, the average slant angle Δa of the surface roughness isan arithmetic average slant angle Δa according to JIS-B-0601, 1998.

In the invention, a stimulating light absorbing layer is suitablyemployable, which is colored so as to absorb the stimulating light. Thestimulating light absorbing layer is a layer containing a colorantselectively absorbing the stimulating light. Such the layer either maybe coated on one side or both sided of the protective layer. Theprotective layer may be colored itself so as to function as thestimulating light absorbing layer.

The protective layer according to the invention may be formed by atransparent organic polymer such as the cellulose derivatives andpoly(methyl methacrylate) or an organic polymer film such as that ofpolypropylene and poly(ethylene terephthalate) so as to have thicknesssuited to the required conveying durability. The organic polymer film ispreferred from the viewpoint of the strength, and a layer of a resincomposition containing fluorinated resin may be coated according tonecessity.

To surely reduce an inroad of moisture into the phosphor sheet havingthe stimulable phosphor layer on the support which is cut into adesignated sized, it is preferable that the circumferences of the waterproof protective films provided on both sides of the phosphor sheet areoutside of the circumference of the phosphor sheet and the protectivefilms are adhered by fusing or a adhesive agent at the area outside thecircumference of the phosphor sheet to form a sealed structure. By suchthe structure, the inroad of moisture from the circumference of thephosphor sheet can be also prevented.

For realizing such the sealed structure, it is preferable that theoutermost surface of the side to be faced to the phosphor layer of thewater proof protective film is constituted by a thermally fusible resin.By such the structure, the films provided on each sides of the phosphorsheet can be adhered by thermal fusion at the area outside thecircumference of the phosphor sheet. Thus the working efficiency ofsealing process can be raised.

The thermally adhesive fusible film in the invention is a resin filmcapable of adhering by thermal fusion, for example, a film ofethylene-vinyl acetate copolymer (EVA), that of polypropylene (PP) andthat of polyethylene (PE). The kind of the film is not limited to theabove examples.

When the fusible adhesive film is employed as the protective film,plural sheets of the film may be laminated to make the suitable moisturepreventing ability adjusting to the required moisture preventingability. Though any known method may be applied for laminating thefilms, a dry lamination method is preferable which is superior in theworking efficiency.

As the method for increasing the average slant angle Δa of the surfaceroughness, a method in which a resin composition coating liquidcontaining fluorinated resin in which an inorganic material such assilica is dispersed is coated on the surface of the protective film, anda method in which plural sheets of film are laminated and the kind offilm to be provided at the outermost surface of the laminated film issuitably selected are employable; but the method is not limited thereto.

Resin films each having various surface states are widely available onthe market, and the film having required average slant angle Δa can beeasily selected.

The surface roughness Ra in μm of the inner side or the phosphor sheetside of the protective layer is an arithmetic average roughness Radefined in the roughness shape parameter of JIS described in JIS-B-0601,1994.

The surface roughness is easily controlled by selection of the roughnessof the resin film to be used or providing a coating layer containing aninorganic substance on the film surface. The coating layer can becolored for making the layer as the stimulating light absorbing layer.Recently, the resin film having an optional surface roughness can beeasily available.

The film such as film of propylene, poly(ethylene terephthalate) andpoly(ethylene naphthalate) has superior properties from the viewpoint ofthe strength as the protective film. However, a part of the incidentstimulating light is repeatedly reflected between the upper and lowerinterfaces of the film and transmitted to the position far from thescanned point since such the film has high refractive index, and thetransmitted stimulating light causes emission of the light from thestimulable phosphor at the position far from the scanned point.Consequently, the sharpness of the image is lowered. Furthermore, thestimulating light reflected by the upper or lower interface of theprotective film in the direction opposite to the surface of the phosphorlayer is further reflected at the light detection device or partsarranged around position, and the reflected light stimulates thestimulable phosphor at the position further far from the scanned pointto cause light emission. Therefore, the sharpness is further lowered.The amount of the stimulating light absorbed in the protective film andthe space in the reading apparatus is small because the stimulatinglight is coherent light having a long wavelength of from red toinfrared. Accordingly, the stimulating light is transmitted to farposition and degrades the sharpness as long as aggressively absorb thescattered and reflected light.

It is supposed that the stimulation light absorbing layer relating tothe invention is effective to inhibit such the scattered and thereflected light.

When the film such as polypropylene film, poly(ethylene terephthalatefilm or poly(ethylene naphthalate) film is used as the constitutingmember of the protective layer according to the invention, theunevenness of the image density or the density variation other than theradiographic image and line-shaped noises can also be reduced. It issupposed that the line-shaped noises are caused in the course of theproduction process of the protective film.

Such the effect is enhanced when the average slant angle Δa is not lessthan 0.01.

Though it is considered that the total reflection of the stimulatinglight at the interface of the protective layer is prevented near suchthe value of the slant angle Δa, such the effect is small when theprotective film has no stimulating light absorbing layer. Accordingly,it is supposed that the above-mentioned effect is the synergistic effectof the light scattering inhibition effect of the stimulating lightabsorbing layer with the total reflection inhibition effect of theaverage slant angle Δa.

By employing the present invention, the use of the protective filmhaving high thermal resistivity and necessary thickness is made possiblewithout degradation of the image quality. Consequently, the radiationimage conversion panel excellent in the thermal resistivity forprolonged period can be realized.

When the resin film is employed as the protective film, the protectivefilm may be constituted by lamination of plural resin films and resinfilms on which a metal oxide is vapor deposited.

In such the case, it is preferable to provide the stimulating lightabsorbing layer between the laminated resin films. By such theconstitution, the stimulating light absorbing layer is protected fromthe physical impact and the chemical degradation and the properties ofthe panel are stably maintained for a long period. It may be either thatthe stimulation light absorbing layers are arranged at plural positionsor that the adhesive layer for laminating the resin films contains acolorant so as to be the stimulating light absorbing layer.

Though the adhesion of the protective film and the phosphor sheet may beperformed by any known method, a method is simple for the productionwork, in which an adhesive agent is previously coated on the side of theprotective film to be contacted to the phosphor sheet and the protectivefilm is adhered by thermal fusion onto the phosphor sheet by the use ofa heating roller.

The shape of the surface of the protective film can be easily controlledby selecting the resin film to be employed or coating a layer containingan inorganic substance on the surface thereof. It is possible to colorthe coating layer to form the stimulating light absorbing layer.Recently, a resin film having an optional surface shape can be easilyavailable.

The method for raising the sharpness by coloring the protective film ofthe radiation image conversion layer is described in Japanese PatentExamined Publication No. 59-23400 as an example of the embodimentsincluding the coloring the support, subbing layer, phosphor layer,interlayer and the protective layer. However, there is neither concretedescription nor suggestion regarding the protective layer.

Colorants capable of absorbing the stimulating light for the radiationimage conversion layer are employed as the colorant to be employed inthe protective film of the radiation image conversion layer according tothe invention.

It is preferable to provide the stimulating light absorbing layer sothat the light transmittance of the protective layer at the wavelengthof the stimulating light is from 98% to 50% of that of the sameprotective layer except that the stimulating layer absorbing layer isomitted. When the ratio is more than 98%, the effect of the invention issmall and when the ratio is less than 50%, the illuminance of theradiation image conversion layer is considerably lowered.

The kind of the colorant to be employed is decided depending on the kindof the stimulable phosphor. As the stimulable phosphor for the radiationimage conversion panel, one emitting light of from 300 to 500 nm bystimulation by stimulating light of from 400 to 900 nm is usuallyemployed. Therefore, blue to green organic or inorganic colorants areusually employed.

Examples of the blue to green organic colorant include Sabon Fast Blue3G manufactured by Hoechst Co., Ltd., Estrol Bril Blue N-3RLmanufactured by Sumitomo Kagaku Co., Ltd., Sumiacryl Blue F-GSLmanufactured by Sumitomo Kagaku Co., Ltd., D & C Blue No. 1 manufacturedby National Aniline Co., Ltd., Spirit Blue manufactured by HodogayaKagaku Co., Ltd., Oil Blue No. 603 manufactured by Orient Co., Ltd.,Quitone Blue A manufactured by Ciba-Geigy Co., Ltd., Aizen Carotin BlueGLH manufactured by Hodogaya Kagaku Co., Ltd., Lake Blue A, F and Hmanufactured by Kyouwa Sangyo Co., Ltd., Rhodarin Blue 6GH manufacturedby Kyouwa Sangyo Co., Ltd., Primo Cyanine 6GX manufactured by InahataSangyo Co., Ltd., Bril Acid Green 6BH manufactured by Hodogaya KagakuCo., Ltd., Cyanine Blue BNRS manufactured by Toyo Ink Co., Ltd., andLyonol Blue SL manufactured by Toyo Ink Co., Ltd. Examples of blue togreen inorganic colorant include Prussian blue, cobalt blue, ceruleanblue, chromium oxide, and TiO₂—ZnO—CoO—NiO type pigments.

As the support to be employed in the radiation image conversion panelaccording to the invention, ones capable of being made as a flexiblesheet or a web are suitable for easiness of handling. From such theviewpoint, a plastic film such as cellulose acetate film, polyesterfilm, poly(ethylene terephthalate) film, poly(ethylene naphthalate)film, polyamide film, polyimide film, triacetate film and polycarbonatefilm is preferred.

Though the thickness of the support may be varied depending on the kindof the material of the support, the thickness is usually from 80 μm to1,000 μm and preferably from 80 μm to 500 μm.

The surface of the support may be either smooth or matted for raisingthe adhesiveness with the stimulable phosphor layer.

On the surface of the support, on which the stimulable phosphor isprovided, a subbing layer may be formed for increasing the adhesivenesswith the stimulable phosphor layer.

Case of coating type stimulable phosphor layer The coating typestimulable phosphor layer is the stimulable phosphor layer of thephosphor sheet formed by coating a coating liquid.

Examples of the binder to be employed in the stimulable phosphor includeproteins such as gelatin, polysaccharides such as dextran, naturalpolymer substances such as gum arabic, and synthesized polymersubstances typically such as poly(vinyl butyral), poly(vinyl acetate),nitrocellulose, ethyl cellulose, vinylidene chloride.vinyl chloridecopolymer, a poly(alkyl acrylate), a poly(alkyl methacrylate), vinylchloride.vinyl acetate copolymer, polyurethane, cellulose acetatebutylate, poly(vinyl alcohol) and a linear polyester.

Among these binder, nitrocellulose, linear polyester, poly(alkylacrylate), poly(alkyl methacrylate), a mixture of linear polyester andpoly(alkyl acrylate) or poly(alkyl methacrylate) and a mixture ofpolyurethane and poly(vinyl butyral) are particularly preferred. Thesebinders may be crosslinked by a crosslinking agent. The stimulablephosphor layer can be formed on the subbing layer by, for example, thefollowing method.

The stimulable phosphor and the binder are added to a suitable solventand sufficiently mixed to prepare a coating liquid composed of solutionof the particles of the binder and that of the stimulable phosphoruniformly dispersed in the binder solution.

The binder is usually employed in a ratio of from 0.01 to 1 part byweight per 1 part by weight of the stimulable phosphor. The amount ofthe binder is preferably to be smaller from the viewpoints of thesensitivity and the sharpness of the radiation image conversion panel,and a ratio of from 0.03 to 0.2 parts by weight is preferable based onthe balance with the easiness of the coating.

Examples of the solvent to be used for preparation of the coating liquidof the stimulable phosphor layer are lower alcohols such as methanol,ethanol, iso-propanol and n-butanol; ketones such as acetone, methylethyl ketone, methyl iso-butyl ketone and cyclohexanone; esters of alower fatty acid and a lower alcohol such as methyl acetate, ethylacetate and n-butyl acetate; ethers such as dioxane, ethylene glycolmonoethyl ether and ethylene glycol monomethyl ether; aromatic compoundsuch as toluene and xylene; halogenized hydrocarbon such as methylenechloride and ethylene chloride; and a mixture the above-mentioned.

Various kinds of additive such as a dispersing agent for improving thedispersing ability of the phosphor in the coating liquid and aplasticizer for binding force between the binder and the phosphor in thestimulable phosphor layer may be mixed with the coating liquid. Examplesof the dispersing agent to be used such the purpose include phthalicacid, stearic acid, caproic acid and lipophilic surfactants. Examples ofthe plasticizer include phosphates such as triphenyl phosphate,tricresyl phosphate and diphenyl phosphate; phthalates such as diethylphthalate and dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethyl glycolate and butyl phthaly butyl glycolate; andpolyesters of poly(ethylene glycol) with an aliphatic dibasic acid suchas polyester of triethylene glycol with adipic acid and polyester ofdiethylene glycol with succinic acid.

The preparation of the stimulable phosphor layer coating liquid isperformed using a dispersing apparatus such as a ball mill, a sand mill,an attriter, a three-roller mill, a high speed impeller disperser, aKady mill and a ultrasonic dispersing apparatus.

The above-prepared coating liquid is coated on the subbing layer to forma coated layer of the coating liquid. The coating can be performed byutilizing usual coating means such as a doctor blade coater, a rollercoater and a knife coater.

The coated layer thus formed is gradually heated to be dried so that theformation of the stimulable phosphor layer is completed. The thicknessof the stimulable phosphor layer is usually from 20 μm to 500 μm, eventhough the thickness may be varied depending on the objective propertiesof the radiation image conversion panel, the kind of the stimulablephosphor and the mixing ratio of the binder to the phosphor. The totalthickness of the stimulable phosphor, subbing layer and the support, ispreferably from 50 μm to 3 mm. Case of gas phase accumulated typestimulable phosphor layer The gas phase accumulated type means that thestimulable phosphor layer of the phosphor sheet is formed byaccumulation of the phosphor in the gas phase by vapor deposition.

<Stimulable Phosphor>

As the stimulable phosphor for forming the stimulable phosphor layer,composite halide phosphors activated by di-valent europium described inJapanese Patent O.P.I. Publication No. 61-236890 are usable. Forexample, iodide-containing rare-earth metal oxohalide compound phosphorseach activated by a rare-earth element, particularly Eu added BaFIcompound are usable. Examples of the stimulable phosphor preferablyemployed in the radiation image conversion panel are phosphorsrepresented by BaSO₄:Ax described in Japanese Patent O.P.I. PublicationNo. 48-80487; phosphors represented by MgSO₄:Ax described in JapanesePatent O.P.I. Publication No. 48-80488; phosphors represented bySrSO₄:Ax described in Japanese Patent O.P.I. Publication No. 48-80489;phosphors of Na₂SO₄, CaSO₄, Na₂SO₄ and BaSO₄, each added with at leastone of Mn, Dy, and Tb described in Japanese Patent O.P.I. PublicationNo. 51-19889; phosphors of BeO, LiF, MgSO₄ and CaF₂ described inJapanese Patent O.P.I. Publication No. 52-30487; phosphors ofLi₂B₄O₇:Cu,Ag described in Japanese Patent O.B.I. Publication No.53-39277; phosphors of Li₂O.(Be₂O₂)_(x):Cu,Ag described in JapanesePatent O.P.I. Publication No. 54-47883; and phosphors represented bySrS:Ce,Sm, SrS:Eu,Sm, La₂O₂S:Eu,Sm, and (Zn,Cd)S:Mn_(x), described inU.S. Pat. No. 3,859,527. Moreover, Phosphors of ZnS:Cu,Pb described inJapanese Patent O.P.I. Publication No. 55-12142; barium aluminatephosphors represented by BaO.x Al₂O₃:Eu and alkali-earth metal silicaterepresented by M(II)O.xSiO₂:A are employable.

Examples of the phosphor preferably used in the invention arefollowings: alkali-earth metal fluoride halide phosphors represented by(Ba_(1-x-y)Mg_(x)Ca_(y))F_(x):Eu²⁺ described in Japanese Patent O.P.I.Publication No. 55-12143; phosphors represented by LnOX:xA described inJapanese Patent O.P.I. Publication No. 55-12144; phosphors representedby (Ba_(1-x)M(II)_(x)F_(x):yA described in Japanese Patent O.P.I.Publication No. 55-12145; phosphors represented by BaFX:xCe described inJapanese Patent O.P.I. Publication No. 55-84389; phosphors of rare-earthmetal-activated di-valent metal fluorohalide represented byM(II)FX.xA:yLn described in Japanese Patent O.P.I. Publication No.55-160078; phosphors represented by ZnS:A, CdS:A or (Zn,Cd)S:A,X:phosphors represented by one of the followings described in JapanesePatent O.P.I. Publication No. 59-38278xM₃(PO₄)₂.NX₂:yAxM₃(PO₄)₂: yA;phosphors represented by one of the followings described in JapanesePatent O.P.I. Publication No. 59-155487nReX₃.mAX′₂:xEunReX₃.mAX′₂:xEu,ySm;and bismuth-activated alkali halide phosphors represented by M(I)X:xBidescribed in 61-228400.

Alkali halide type stimulable phosphors represented by the followingFormula 1 such as those described in Japanese Patent O.P.I. PublicationNos. 61-72087 and 2-58000 are particularly preferred.M1X.aM2X′₂.bM3X″₃:eA   Formula 1

In the above formula, M1 is an alkali metal selected from the groupconsisting of Li, Na, K, Rb and CS; M2 is a di-valent metal selectedfrom the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu and Ni; M3is a tri-valent metal selected from the group consisting of Sc, Y, La,Ce, Pr, Nd, Pm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga and In; X, X′and X″ are each a halogen selected from the group consisting of F, Cl,Br and I; A is a metal selected from the group represented by the groupconsisting of Eu, Tb, In, Ga, Cs, Tm, Dy, Pr, Ho, Nd, Yb, Er, Gd, Lu,Sm, Y, Tl, Na, Ag, Cu and Mg; and a, b and e are each a value within therange of 0≦a<0.5, 0≦b<0.5 and 5.0<e≦0.2, respectively.

In Formula 1, M1 is preferably selected from the group consisting of K,Rb and Cs and X is preferably elected the group consisting of from Brand I.

M2 is preferably selected from the group consisting of Be, Mg, Ca andBa; M3 is preferably selected from the group consisting of Y, Ce, Sm,Eu, Al, La, Gd, Lu, Ga and In; b is preferably 0≦b≦0.01; and A ispreferably selected from the group consisting of Eu, Cs, Sm, Tl and Na.

These alkali halide type phosphors each forms slender columnar crystalson the substrate by a gas phase accumulation method, which have a slantof certain angle with the substrate. The angle of the crystals may bevertical to the substrate. It is a feature of the use of such thephosphor that the sharpness of the image is high because the diffusionof the stimulating light or the emitted light in the sideway directionis inhibited by the formation of such the columnar crystals. Among thealkali halide type stimulable phosphors, RbBr and CsBr type phosphor arepreferable since they give high illuminance and high image quality.

In the invention, the phosphors represented by the following Formula 2are particularly preferred.CsX:A   Formula 2

In the formula, Br is Br or I, and A is Eu, In, Ga or Ce.

Among them, CsBr type phosphors are preferable since they emit highilluminance light and give a high quality image, and shows highimproving effect on the adhesiveness with the substrate according to theproducing method relating to the invention.

In the invention, the preferable columnar crystals in which the crystalsare individually grown in a columnar shape having certain space betweenthem, can be formed by the method described in Japanese Patent O.P.I.Publication No. 2-58000.

The stimulable phosphor layer composed of independent slender columnarcrystals can be obtained by the gas phase growing or accumulation methodsuch as a vapor deposition method.

For example, the columnar crystals approximate vertically grown from thesubstrate can be obtained by hitting the vapor stream to the substratewith an angle of from 0° to 5° with the substrate surface.

In such the case, it is suitable that the shortest distance between thesubstrate and the crucible is set about from 10 cm to 60 cm for suitingthe average range of the vapor of the stimulable phosphor.

The stimulable phosphor as the evaporation source is charged in thecrucible in uniformly molted state or in a shaped state by pressing orhot pressing. A degas treatment is preferably applied on this occasion.Though the evaporation of the stimulable phosphors from the evaporationsource is carried out by scanning by an electron beam generated by anelectron gun, the evaporation may be performed by another method.

It is necessary not always that the evaporation source is the stimulablephosphor, it may be a mixture of the raw materials of the stimulablephosphor.

{0079}

The activator may be added by depositing the mixture of the basicsubstance and the activator or doping the activator after the depositionof the basic substance. For example, in the case of CsBr is employed asthe basic substance, CsBr is solely vapor deposited and In is doped asthe activator. The doping is possible even when the thickness of thelayer is large since the crystals are each independent and the MTF isnot lowered because the growing the crystals is difficultly occur.

The doping can be performed by thermal diffusion or ion injection intothe layer of the basic substance of the phosphor.

<Thickness of Phosphor Layer and the Size of Crystal>

The thickness of the layer constituted by the columnar crystals formedby the above-described method is preferably 50 μm to 1,000 μm and morepreferably from 50 μm to 800 μm even though the thickness may be variedaccording to the sensitivity to the radiation of the objective radiationconversion panel and the kind of the stimulable phosphor.

For improving the modulation transfer function (MTF) of the stimulablephosphor layer composed of the columnar crystals, the size of thecolumnar crystal is preferably about from 0.5 to 50 μm and morepreferably from 0.5 to 20 μn. The size of the crystals is the averagevalue of the circle-equivalent diameter of each of the columnar crystalsobserved from the face parallel with the substrate, which is determinedby a microscopic photograph including at least 100 of the columnarcrystals. When the columnar crystal is thinner than 0.5 μm, the emittedlight is scattered by the crystals and lowering of the MTF is resulted,and when columnar crystal is thicker than 50 μm, the directivity of theemitted light is lowered and lowering of the MTF is also resulted.

For gas phase growing or accumulating the stimulable phosphor, the vapordepositing method, spattering method and chemical vapor deposition (CVD)method are applicable.

In the vapor deposition method, the substrate or support is set in avapor deposition apparatus and air in the apparatus is exhausted whileintroducing inactive gas from the introducing opening to make the vacuumdegree to about 1.333 Pa to 1.33'10⁻³ Pa. And then at least one of thestimulable phosphor is heated and evaporated by a resistor heatingmethod or an electron beam method to accumulate the phosphor untildesignated thickness. Thus the stimulable phosphor layer containing nobinder is formed. It is possible in the vapor deposition process thatthe formation of the stimulable phosphor layer is separated pluraltimes. In the vapor deposition process, plural resistor heating devicesor electron beams may be applied to perform the vapor deposition. It isalso possible in the vapor deposition method that the raw materials ofthe stimulable phosphor are evaporated by plural resistor heatingdevices or electron beams so that the objective stimulable phosphor issynthesized on the support while forming the stimulable phosphor. Thesupport or substrate may be heated or cooled according to necessity inthe vapor deposition method. A heating treatment may be applied afterthe deposition.

In the spattering method, the substrate is set in a spattering apparatussimilar in the case of the vapor deposition method, and air in theapparatus is exhausted and then inactive gas such as Ar and Ne isintroduced in the apparatus to make the pressure in the apparatus toabout 1.33 Pa to 1.33×10⁻³ Pa. And then spattering is performed on thestimulable phosphor as the target to accumulate the phosphor on thesubstrate to a desired thickness. In the spattering process, thespattering can be separated into plural times as the same as in thevapor deposition method. The stimulable phosphor layer may be formed byspattering the target using each of them. It is also possible in thespattering method to form the objective stimulable phosphor layer on thesubstrate by successively or simultaneously spattering plural rawmaterials of the stimulable phosphor, and reactive spattering may beperformed by introducing gas such as O₂ or H₂ according to necessity. Inthe spattering method, the substrate may be heated or cooled on theoccasion of the spattering according to necessity. A heating treatmentmay be applied after the spattering.

The CVD method is a method in which the stimulable phosphor or anorganic metal compound is decomposed by energy such as heat or highfrequency electric power to form the stimulable phosphor layer withoutbinder on the substrate. By such the method, the independent slendercolumnar crystals of the stimulable phosphor can be grown at adesignated angle with the normal line direction of the substrate in gasphase.

These columnar crystals can be obtained by the method described inJapanese Patent O.P.I. Publication No. 2-58000 in which the stimulablephosphor or the raw materials thereof are supplied in a state of vaporto grow or accumulate the crystals in gas phase.

FIG. 1 shows the situation of formation of the stimulable phosphor layeron the support 11 by the vapor deposition. In the drawing, θ₂ is theincident angle of the vapor stream 16 of the stimulable phosphor withthe normal line direction R of the support and θ₁ is the angle of thecolumnar crystal with the normal line direction R of the support; thecolumnar crystal 13 is grown at this angle θ₁. In the drawing, θ₂ is 60°and θ₁ is approximately 30°; empirically, the growing angle θ₁ becomesabout half of the incident angle θ₂.

The stimulable phosphor layer formed on the support 11 by such themethod is excellent in the directivity of the stimulating light and theemitted light since it contains no binder. Consequently, the thicknessof the layer can be made thicker than that of the radiation imageconversion panel having a dispersion type stimulable phosphor layer inwhich the stimulable phosphor is dispersed in the binder. Furthermore,scattering of the emitted light in the stimulable phosphor layer isreduced so that the sharpness of the image improved.

The space between the columnar crystals may be filled by filler tostrengthen the stimulable phosphor layer. A substance having high lightabsorbing ability and that having a high reflective index may be filledin the space between the columnar crystals. Such the filling iseffective to strengthen of the layer and to reduce of the diffusing ofthe incident stimulating light in the sideways direction.

The high reflective substance is a substance having high reflectivity tothe stimulating light of from 500 to 900 nm, particular from 600 to 800nm, for example, metals such as aluminum, magnesium, copper and indium,white pigments and colorants of green to red region can be employed.

The white pigment can also reflect the stimulating light. Examples ofthe white pigment are anatase and rutile type TiO₂, MgO, PbCO₃.Pb(OH)₂,C, Al₂O₃, M(II)FX in which M(II) is at least one of Ba, Sr and Ca and Xis at least one of Cl and Br, CaCO₃, ZnO, Sb₂O₃, SiO₂, ZrO₂, lithopone(BaO₄.ZnS), magnesium silicate, basic lead silicate-sulfate, basic leasphosphate and aluminum silicate. These white pigments easily scatter theemitted light by reflecting and refracting the light since therefracting index thereof is high and the sensitivity of the radiationimage conversion panel thus obtained is considerably increased.

As the substance with high absorbency, for example, carbon, chromiumoxide, nickel oxide, iron oxide and blue colorants are employed. Carbonabsorbs the emitted light also.

The colorant is may be either an inorganic or an organic corolant.Examples of the organic colorant are Sabon Fast Blue 3G manufactured byHoechst Co., ltd., Estrol Bril Blue N-3RL manufactured by SumitomoKagaku Co., ltd., D & C Blue No. 1 manufactured by National Aniline Co.,ltd., Spirit Blue manufactured by Hodogaya Kagaku Co., ltd., Oil BlueNo. 603 manufactured by Orient Co., ltd., Quitone Blue A manufactured byCiba-Geigy Co., ltd., Aizen Carotin Blue GLH manufactured by HodogayaKagaku Co., ltd., Lake Blue AFH manufactured by Kyouwa Sangyo Co., ltd.,Primo Cyanine 6GX manufactured by Inahata Sangyo Co., ltd., Bril AcidGreen 6BH manufactured by Hodogaya Kagaku Co., ltd., Cyan Blue BNRCSmanufactured by Toyo Ink Co., ltd., and Leionoyl Blue SL manufactured byToyo Ink Co., ltd. Organic metal complex colorants such as Color IndexNos. 24411, 23160, 74180, 74200, 22800, 23154, 23155, 24401, 14830,15050, 15760, 15707, 17941, 74220, 13425, 13361, 13420, 11836, 74140,74380, 74350 and 74460 are also usable.

As the inorganic colorants, Prussian blue, cobalt blue, cerulean blue,chromium oxide and TiO₂—ZnO—Co—NiO type pigments are cited.

The thickness of the stimulable phosphor layer of the radiation imageconversion panel is preferably from 10 μm to 1,000 μm and morepreferably from 10 μm to 500 μm even though the thickness can be varieddepending on the characteristics of the objective radiation imageconversion panel, the kind of the stimulable phosphor and the mixingratio of the binder and the stimulable phosphor.

The phosphor sheet composed of the support on which the phosphor layeris provided was cut into a designated size. Though the cutting may beperformed by any method, the use of a trimming machine or a punchingmachine is desirable from the viewpoints of the working efficiency andthe cutting accuracy.

EXAMPLES

The invention is described in detail below referring examples; theembodiment is not limited to the examples.

Example 1

<<Preparation of Radiation Image Conversion Panel>>

(Preparation of Coated Type Stimulable Phosphor Layer)

To methyl ethyl ketone, 200 g of stimulable phosphor(BaFBr_(0.85)I_(0.15):Eu²⁺ _(0.001)), 8.0 g of polyurethane resin(Pandex T5265 manufactured by Dainihon Ink Kagaku Kogyo Co., Ltd.) and2.0 g of epoxy resin (EP1001 manufactured by Yuka Shell Epoxy Co., Ltd.)as a yellowing preventing agent were added and dispersed by a propellermixer to prepare a coating liquid for forming a phosphor layer having aviscosity of 30 Pa.s at 25° C. The coating liquid was coated onpoly(ethylene terephthalate) film having a thickness of 300 μm anddried. Thus a stimulable phosphor sheet which had a stimulable phosphorlayer having a thickness of 230 μm was obtained.

(Preparation of Gas Accumulated Type Stimulable Phosphor Layer)

On the surface of a crystalline glass palate, manufactured by NihonDenki Glass Co., Ltd., having a size of 410 mm×410 mm and a thickness of1 mm, a stimulable phosphor layer comprising a stimulable phosphor ofCsBr:Eu was formed by employing the gas phase accumulation apparatusshown in FIG. 2.

On the occasion of the vapor deposition, the support was set in the gasphase accumulation apparatus and the raw material of CsBr:Eu shaped bypressing was charged into a crucible cooled by water, not shown in thedrawing, as the evaporation source.

Thereafter, a pump was connected to the exhausting opening and air inthe apparatus was exhausted, and nitrogen was introduce from the gasintroducing opening with a introducing rate of 1,000 sccm (sccm:standard cc/min or 1×10⁻⁶ m³/min) to maintain the vacuum degree in theapparatus at 6.65×10⁻³ Pa. Then the evaporation source was heated by650° C. so that the alkali halide of CsBr:Eu_(0.0001) layer was formedon a side of the support. On the occasion of the vapor deposition, theevaporation source was positioned on the normal line of the supportsurface, namely θ₂ was about 0°, and the distance d between the supportand the evaporation source was set at 60 cm. The deposition was carriedout while the support was conveyed in the direction parallel with thesupport. The deposition was finished when the thickness of thestimulable phosphor layer is attained at 400 μm to prepare a sample ofthe stimulable phosphor sheet.

(Preparation of Protective Film 1)

Poly(ethylene terephthalate) (PET) films each having the average slantangle shown in Tale 1 were used as the protective film of the phosphorsheet. On the side of the PET film an adhesive layer of 1 μm wasprovided by coating an adhesive agent, Vylon 300 manufactured by ToyoboCo., Ltd.

On the other side of the PET film, emitted light absorbing layers havingvarious surface roughnesses as shown Table 1 were coated by thefollowing method.

(Preparation of Emitting Light Absorbing Layer)

To methyl ethyl ketone, 50 g of fluorinated resin (50 weight percentxylene solution of fluoroolefin-vinyl ether copolymer Lumifron LF100manufactured by Asahi glass Co., Ltd.), 5 g of a crosslinking agent ofisocyanate (Coronate HX manufactured of Nihon Polyurethane Co., Ltd.,having a solid content of 100% by weight) and 0.5 g of alcohol-modifiedsilicone oligomer having a skeleton of dimethylpolysiloxane and hydroxylgroups (carbinol groups) at both of the terminals (X-22-2809manufactured by Shin'etsu Kagaku Kogyo Co., Ltd.) were added to preparea coating liquid having a viscosity of from 0.1 to 0.3 Pa.s. To thecoating liquid, a previously prepared dispersion of a mixture of anorganic blue colorant (Sabon Fast Blue 3G manufactured by Hoechst Co.,Ltd.) and silica having a diameter of from 0.2 to 2.0 μm was added. Andthen the coating liquid was coated on the surface of the PET film andthe coated layer was thermally hardened by a heat treatment at 120° C.for 20 minutes to form an emitting light absorbing layer.

On this occasion, emitted light absorbing layers each having optionalsurface roughness and the light transmittance were prepared bycontrolling the adding amount of the colorant and the silica. The lighttransmittance of the emitting light absorbing layer is a relative valueof the transmittance to light of 633 nm generated by a He—Ne laser tothat of the protective layer the same as the sample except that theemitted light absorbing layer is omitted.

(Protective Film 2)

As the protective film for the phosphor layer side of the phosphorsheet, a film having the following constitution A was used.

Constitution A

Matted Film/VMPET 12/Sealant Film 30

In the above:

-   -   Matted film: various films having matted surface (available on        the market)    -   VMPET film: Alumina deposited PET film (Toyo Metalizing Co.,        Ltd., available on the market)    -   Sealant film: Thermal fusion adhesive film made by casting        polypropylene (CPP)

The number attached after the resin film name is the thickness of thefilm in μm.

In the above, “/” means the presence of a layer of an adhering agenthaving a thickness of 2.5 μm. The adhering agent was two-liquid typereactive urethane adhering agent for dry lamination.

The entire layers of the adhering agent were made as emitted lightabsorbing layers by adding an organic blue colorant, Sabon Fast Blue 3Gmanufactured by Hoechst Co., Ltd., previously dispersed and dissolved inmethyl ethyl ketone.

Laminated protective films each having various surface roughness wereprepared by controlling the average slant angle Δa of the surfaceroughness by changing the kind of the matted film and by selecting thesurface roughness Ra of the sealant film.

(Protective Film for Support Side of Phosphor Sheet: Preparation ofBackside Protective Film)

The protective film for the support side of the phosphor sheet was a drylaminated film having the constitution of

-   -   sealant film/aluminum foil film/poly(ethylene terephthalate)        film of 188 μm. The thickness of the adhering agent was 1.5 μm        and the adhering agent was two-liquid type reactive urethane        adhesive agent.        (Sealing of Phosphor Sheet)

The above prepared phosphor sheets were each cut into a square of 45cm×45 cm and sandwiched between the protective films 1 and 2, and thecircumference of them was fused and adhered by an impulse sealer in anatmosphere of reduced pressure.

The distance of from the adhered portion to the edge of the phosphorsheet was 1 mm. The width of the heated of the impulse sealer was 8 mm.

Radiation image conversion panels 1 through 21 were prepared asabove-described.

<<Evaluation of Radiation Image Conversion Panel>>

Radiation conversion panels 1 through 21 prepared as above weresubjected to the following evaluations.

1) Evaluation of Sharpness

The sharpness was evaluated as follows:

The radiation image conversion panel was exposed to X-ray of 80 kVp ofbulb voltage through a MTF chart of lead and then stimulated by He—Nelaser light of 633 nm to emit light. The light emitted from the phosphorsheet was received by a light receiving device, a photomultiplier havingspectral sensitivity S-5, and converted to electric signals. Theelectric analogue signals were converted to digital signals and recordedin a hard disc. The record was analyzed by a computer to determine MTFof the X-ray image. In Table 1, the MTF values at a space frequency of 1cycle/mm are shown. Higher value of the MFT corresponds to highersharpness.

2) Evaluation of Unevenness of Image Density and Line-Shaped Noise

The radiation image conversion panel was exposed to X-ray of 80 kVp ofbulb voltage and then stimulated by He—Ne laser light of 633 nm to emitlight. The light emitted from the phosphor sheet was received by a lightreceiving device, a photomultiplier having spectral sensitivity S-5, andconverted to electric signals. The signals was reproduced by an imagereproducing apparatus and printed out as an image enlarged by to timesby an output apparatus. Occurrence of the unevenness of the imagedensity and the line-shaped noise were evaluated by visual observationof the printed out image. The unevenness of image density and theline-shaped noise were classified according to the following norms andshown in Table 1.

The samples subjected to accelerated test in an oven at 80° C. for 2days were evaluated in the same manner as above for evaluating thethermal resistivity.

-   A: Unevenness of image density and the line-shaped noise are not    observed at all.-   B: One or two weak unevenness of image density or line-shaped noise    are observed in the image.-   C: Three or four weak unevenness of image density or line-shaped    noise are observed in the image.-   D: Though five or more unevenness of image density or line-shaped    noise are observed in the image, the dark portion is four or less.-   E: Five or more dark unevenness of image density or line-shaped    noise are observed in the image.

The evaluation results are listed in Table 1.

It is clear from Table 1 that the radiation image conversion panelsaccording to the invention are lower in the occurrence of the unevennessof image density and the line-shaped noise compared with the comparativeradiation image conversion panels. TABLE 1 Average slant angle Surfaceroughness Emitted Unevenness of image Δa of outer side Ra (μm) of innerlight Trans- density, line-shaped noise Panel Protective Phosphorsurface of side of the absorbing mittance Sharpness After 2 days No.film sheet protective layer protective layer layer (%) (%) Initial timeat 80° C. Remarks 1 1 *1 0.006 0.50 Without 99.5 61 E E Comp. 2 1 *10.010 0.45 Without 99.5 64 D D Inv. 3 1 *1 0.017 0.30 Without 99.5 64 CC Inv. 4 1 *1 0.006 0.55 With 98.0 69 D E Comp. 5 1 *1 0.010 0.45 With98.0 75 B B Inv. 6 1 *1 0.014 0.20 With 95.0 76 A A Inv. 7 1 *1 0.0190.40 With 90.0 79 A A Inv. 8 1 *1 0.050 0.10 With 80.0 80 A A Inv. 9 1*1 0.100 0.05 With 70.0 81 A A Inv. 10 1 *2 0.006 0.50 With 95.0 79 C DComp. 11 1 *2 0.017 0.30 With 95.0 80 A A Inv. 12 2 *1 0.006 0.50Without 99.5 62 E E Comp. 13 2 *1 0.010 0.45 Without 99.5 65 D D Inv. 142 *1 0.006 0.55 With 98.0 70 D E Comp. 15 2 *1 0.010 0.45 With 98.0 77 BB Inv. 16 2 *1 0.014 0.20 With 95.0 77 A A Inv. 17 2 *1 0.019 0.40 With90.0 80 A A Inv. 18 2 *1 0.050 0.10 With 80.0 81 A A Inv. 19 2 *1 0.1000.05 With 70.0 82 A A Inv. 20 2 *2 0.006 0.50 With 95.0 79 C D Comp. 212 *2 0.017 0.30 With 95.0 81 A A Inv.*1; Coating provided type*2; Gas phase accumulation typeComp.; ComparativeInv.; Inventive

1. A radiation image conversion panel comprising a phosphor sheet havinga stimulable phosphor layer and a protective layer provided so as tocover the surface of the phosphor layer, wherein the average slant angleΔa of the surface roughness of the outer surface of the protective layernot adjacent with the phosphor layer is from 0.01 to 0.1 and the surfaceroughness Ra in μm of the inner side or the phosphor sheet side of theprotective layer is 0.05 μm to 0.45 μm, and the stimulable phosphorlayer is provided by a coating provided type.
 2. The radiation imageconversion panel of claim 1 wherein the protective layer comprises astimulating light absorbing layer being colored to absorb thestimulating light.
 3. The radiation image conversion panel of claim 1,wherein a binder to be used in the stimulable phosphor layer is selectedfrom proteins such as gelatin, polysaccharide such as dextrin, naturalpolymers such as gum arabic, and synthesized polymers such as poly(vinylbutyral), poly(vinyl acetate), nitrocellulose, ethyl cellulose,vinylidene chloride-vinyl chloride copolymer, poly(alkyl acrylate),poly(alkyl methacrylate), vinyl chloride-vinyl acetate copolymer,polyurethane, cellulose acetate butylate, poly(vinyl alcohol) and linearpolyester.
 4. The radiation image conversion panel of claim 3, whereinthe binder is selected from nitrocellulose, linear polyester, poly(alkylacrylate), poly(alkyl methacrylate), a mixture of nitrocellulose andlinear polyester, a mixture of nitrocellulose and poly(alkyl acrylate)or poly(alkyl methacrylate) and a mixture of polyurethane and poly(vinylbutyral).
 5. The radiation image conversion panel of claim 1, whereinthe ratio of the amount of the binder to be used in the stimulablephosphor layer to that of the stimulable phosphor is within the range offrom 0.03 to 0.2 parts by weight per one part by weight of thestimulable phosphor.
 6. The radiation image conversion panel of claim 1,wherein the panel is composed of the phosphor sheet cut into adesignated size and moisture proof films provided on both sides of thephosphor sheet and the films are each substantially not adhered with thephosphor sheet and the circumferences thereof are arranged at theoutside of the circumference of the phosphor sheet so as to cover theentire surface of the phosphor sheet.
 7. A radiation image conversionpanel comprising a phosphor sheet having a stimulable phosphor layer anda protective layer provided so as to cover the surface of the phosphorlayer, wherein the average slant angle Δa of the surface roughness ofthe outer surface of the protective layer not adjacent with the phosphorlayer is from 0.01 to 0.1 and the surface roughness Ra in μm of theinner side or the phosphor sheet side of the protective layer is 0.05 μmto 0.45 μm, and the stimulable phosphor layer is provided by a gas phaseaccumulation type.
 8. The radiation image conversion panel of claim 7,wherein the protective layer comprises a stimulating light absorbinglayer being colored to absorb the stimulating light.
 9. The radiationimage conversion panel of claim 7, wherein the stimulable phosphorconstituting the stimulable phosphor layer is alkali halide typestimulable phosphor represented by Formula 1:M1X.aM2X′₂.bM3X″₃:eA   Formula 1 wherein M1 is an alkali metal selectedfrom the group consisting of Li, Na, K, Rb and Cs; M2 is a divalentmetal selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd,Cu and Ni; M3 is a trivalent metal selected from the group consisting ofSc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al,Ga and In; X, X′ and X″ are each a halogen selected from the groupconsisting of F, Cl, Br and I; A is a metal selected from the groupconsisting of Eu, Tb, In, Cs, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Gd, Lu,Sm, Y, Tl, Na, Ag, Cu and Mg; and a, b and e are each a value within therange of 0≦a<0.5, 0≦b<0.5 and 0<e≦0.2, respectively.
 10. The radiationimage conversion panel of claim 9, wherein the stimulable phosphorconstituting the stimulable phosphor layer is represented by Formula 2:CsX:A   Formula 2 in which X is Br or I, and A is Eu, In, Ga or Ce. 11.The radiation image conversion panel of claim 10, wherein the stimulablephosphor constituting the stimulable phosphor layer is CsBr typephosphor.
 12. The radiation image conversion panel of claim 7, whereinthe panel is composed of the phosphor sheet cut into a designated sizeand moisture proof films provided on both sides of the phosphor sheetand the films are each substantially not adhered with the phosphor sheetand the circumferences thereof are arranged at outside of thecircumference of the phosphor sheet so as to cover the entire surface ofthe phosphor sheet.